1. Field of Invention
The invention relates to an electrophotographic image forming apparatus, such as a laser printer.
2. Description of Related Art
Electrophotographic image forming apparatus are well known in the art. These devices, such as a laser printer, typically includes a photosensitive drum, a charger, a laser scanner, a developing roller, and a transfer roller. After the surface of the photosensitive drum is uniformly charged by the charger, the surface of the photosensitive drum is irradiated with a laser beam emitted from the laser scanner, and an electrostatic latent image is formed based on predetermined image data.
Toner carried on the developing roller is supplied to the electrostatic latent image formed on the surface of the photosensitive drum. The toner deposited on the surface of the photosensitive drum is transferred to a sheet passing between the photosensitive drum and the transfer roller.
Transfer bias is applied to the transfer roller to transfer the toner to the sheet. Either constant-voltage control or constant-current control is employed to control the transfer bias. It is difficult to arrive at a compromise between the constant-voltage control and the constant-current control because both methods have advantages and disadvantages.
The invention provides an image forming apparatus and method where a transfer bias is applied to a transfer roller without causing a transfer failure of toner to a sheet when the resistances of the transfer roller and the sheet change in accordance with a change in temperature, humidity, and size of the sheet.
When the transfer bias is controlled by the constant-voltage control alone, the current value changes as the resistances of the transfer roller and the sheet change with changes in environmental factors such as temperature and humidity and with changes in type and size of the sheet. For example, when the resistance increases, a shortage of the transfer current occurs, which results in a transfer failure. However, when the transfer bias is controlled by the constant-current control alone, a constant transfer current can be supplied at all times even when the resistances of the transfer roller and the sheet change with changes in environmental factors such as temperature and humidity and changes in the type and size of the sheet. Therefore, it is preferable that the transfer bias is applied to the transfer roller under constant-current control.
If the sheet is substantially as wide as the transfer roller when the transfer bias is applied under constant-current control alone, a constant transfer bias is supplied at all times even when the resistances of the transfer roller and the sheet change. Thus, the sheet can be uniformly charged. However, if the sheet is narrower than the transfer roller, the transfer roller makes, direct contact at both ends with the photosensitive drum without a sheet therebetween. The amount of transfer current flowing directly from the ends of the transfer roller to the photosensitive drum relatively increases, and a shortage of the transfer current occurs, which results in a transfer failure. If the constant-current control is switched to the constant-voltage control when the resistances of the transfer roller and the sheet decrease, a transfer failure can be prevented. However, abrupt switching from the constant-current control to the constant-voltage control would cause a drastic increase in the transfer current amount. In such a case, the surface voltage of the photosensitive drum becomes unstable and the image quality deteriorates. If an excessive amount of transfer current flows to the surface of the photosensitive drum, the photosensitive layer may be damaged.
In recent years, a discharge lamp, which reduces the surface voltage of the photosensitive drum after the toner transfer, has been omitted from many laser printers to simplify their structure and reduce their manufacturing cost. In this case, the surface voltage of the photosensitive drum becomes more unstable. Alternatively, the transfer bias can be changed based on the temperature and humidity detected by a sensor within the laser printer. In this case, however, the laser printer becomes complicated in structure and the manufacturing cost thereof is increased.
According to the invention, an image forming device has a transferring element that transfers toner, forming a visualized image on a photosensitive member, to a recording medium and a transfer bias application device that applies a transfer bias to the transferring element. Under the control of the transfer bias application device, the transfer bias is outputted at a constant current value so that a transfer current required for transferring the toner to a recording medium is obtained. In this case, when the transfer current passing though the recording medium decreases, the constant current value of the transfer bias outputted is set to be higher in order to increase the current passing through the recording medium.
Normally, the transfer bias application device performs the constant-current control where the transfer bias set at a constant current value is applied to the transferring element. Accordingly, even if the resistances of the transferring element and the recording medium change with changes in temperature and humidity within a certain range and with changes in type and size of the recording medium within a certain range, a constant transfer current is supplied to the recording medium at all times to sufficiently charge the recording medium.
When the size of the recording medium is smaller than the width of the transferring element, a relatively large amount of current flows directly from the transferring element to the photosensitive member and a relatively small amount of current flows through the recording medium. When the resistances of the transferring element and the recording medium decrease and particularly when the resistance of the transferring element is lower than that of the recording medium, the amount of current flowing through the recording medium tends to decrease. In this case, the recording medium is not sufficiently charged and, as a result, the toner is not transferred from the photosensitive member to the recording medium. In this state, the transfer bias application device increases the current value of the transfer bias outputted from the transfer bias application device to increase the amount of current flowing through the recording medium.
When the amount of current flowing through the recording medium decreases, the transfer bias application device gradually increases the current value instead of performing constant-current control. When the control method is changed, the transfer current does not increase drastically. Accordingly, fluctuations in the surface voltage of the photosensitive member and a damage to a photosensitive layer are effectively prevented. In addition, because the above-described control is performed without a temperature/humidity sensor, the image forming device can be simplified in structure and the manufacturing cost thereof can be reduced.
When the transfer bias application device gradually increases the current value, the output voltage from the transfer bias application device is kept constant and the amount of transfer current is increased as a linear function with the decrease in the resistance of the recording medium and the transferring element. Under the above-described control, the amount of current can be easily adjusted and the transfer bias can be reliably applied to the recording medium.
The transfer bias application device includes a constant-current control circuit that keeps constant the current value of the transfer bias applied to the transferring element, a variable-current control circuit that changes the current value of the transfer bias applied to the transferring element in response to changes in the resistances of the recording medium and the transferring element, and a transfer bias output circuit that outputs the transfer bias. The transfer bias output circuit is controlled by either the constant-current control circuit or the variable-current control circuit and outputs the transfer bias as controlled.
The constant-current control circuit supplies a predetermined input to the transfer bias output circuit. The predetermined input changes in response to the resistances of the transferring element and the recording medium and is used to output a constant current value from the transfer bias output circuit at all times. In contrast, the variable-current control circuit supplies a constant input to the transfer bias output circuit. The transfer bias control circuit outputs the transfer bias based on the input from the variable current control circuit. The current value of the transfer bias is not constant and varies depending on the total resistance including the resistance of the transferring element and that of the recording medium.
Upon the application of the transfer bias to the transferring element and upon the flow of transfer current to the transferring element, a voltage is generated in the transferring element in response to the transfer current value and the total resistance including the resistance of the transferring element and that of the recording medium. This voltage is defined as a transfer voltage. During the constant-current control, the transfer voltage changes in response to changes in the resistances. When the absolute value of the transfer voltage decreases below a predetermined level with decrease in the resistances, the control by the constant-current control circuit is switched to the control by the variable-current control circuit. The predetermined level is previously determined during the device design phase and is reflected on an input from the variable-current control circuit to the transfer bias output circuit. Which control is employed to control the transfer current is determined by comparison between an input from the constant-current control circuit to the transfer bias application circuit and an input from the variable-current control circuit to the transfer bias application circuit.
These inputs are voltages. When the input voltage from the constant-current control circuit to the transfer bias application circuit is higher, the transfer bias outputted from the transfer bias output circuit is subjected to constant-current control. When the input from the variable-current control circuit to the transfer bias application circuit is higher, the current value of the transfer bias outputted from the transfer bias output circuit is changed. The transfer bias current control method is switched depending on the high-low relationship of the inputs to the transfer bias control circuit. Accordingly, the transfer bias can be reliably controlled with a very simple structure and at a reduced cost.
In addition, a diode is connected between the constant-current control circuit and the transfer bias application circuit, and another diode is connected between the variable-current control circuit and the transfer bias application circuit. Thus, when the output voltage from the constant-current control circuit becomes higher than the output voltage from the variable-current control circuit, no current flows from the constant-current control circuit to the variable-current control circuit. When the output voltage from the variable-current control circuit becomes higher than the output voltage from the constant-current control circuit, no current flows from the variable-current control circuit to the constant-current control circuit, either. As the transferring element, it is preferable to use a transfer roller and more preferable to use an ionic conduction type transfer roller.
Toner is transferred to the recording medium by contacting the recording medium to the transfer roller and by applying a voltage of the opposite polarity to that of the toner to the transfer roller. Because the transfer roller makes contact with the recording medium, the recording medium itself is charged to a lesser extent and easily removed from the transfer roller. An ionic conduction type transfer roller is advantageous in that its resistance is uniform and varies slightly, but disadvantageous in that its resistance varies greatly with environmental factors such as temperature and humidity. According to the invention, a current required for the toner transfer is reliably obtained even when the resistance of the transfer roller varies.
The constant current value of the transfer bias, which is controlled by the constant-current control circuit and outputted from the transfer bias, can be made selectable depending on the size of the recording medium. This prevents a shortage of the transfer current caused by variations in size and ensures a sufficient supply of the transfer current to the recording medium of any size.
When the recording medium is small in size, the amount of current flowing directly from the transferring element to the photosensitive member increases and the amount of current flowing through the recording medium decreases. For this reason, when the recording medium is small, the current value of the transfer bias outputted from the transfer bias output circuit is controlled to be at a higher value. Thereby, the recording medium can be sufficiently charged and the toner can be reliably transferred to the recording medium.
Particularly, operability of the image forming device is improved if it is structured to detect the size of the recording medium and change the current value by the detected size.
While this invention has been described in conjunction with specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.